1. Field of the Invention
The present invention relates to an electrowetting device utilizing an electrowetting (electrocapillary) phenomenon, and a varifocal lens, an optical pickup device, an optical recording/reproduction device, a droplet operation device, an optical element, a zoom lens, an imaging device, a light modulating device, and a display device using the same.
2. Description of the Related Art
Varioptic S.A. of France and Royal Philips Electronics N.V. of the Netherlands disclose varifocal lens devices, for example, as electrowetting devices utilizing electrowetting phenomena (see Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2001-519539 and S. Kuiper et al., “Variable-focus liquid lens for miniature cameras”, Applied Physics Letters, Vol. 85, No. 7, 16 Aug. 2004, pp. 1128-1130, for example).
A varifocal lens described in the aforementioned Japanese Unexamined Patent Application Publication and Applied Physics Letters will be described with reference to schematic cross-sectional views of FIGS. 1A and 1B.
The varifocal lens basically includes a container 310 having a cylindrical shape or the like; a conductive liquid material 301 packed in the container 310; and an insulating liquid material 302 made of a material not mixed with the conductive liquid material 301 and packed in the container 310. The conductive liquid material 301 and the insulating liquid material 302 are both light transmissive, have different refractive indices, and have an equal density (specific gravity). In this example, first electrodes 305 are each continuously formed on inner walls, one edge, and outer surfaces of the container, and dielectric films 304 are formed inside the inner walls. The edge of the container 310 covered with the first electrodes 305 is fluid-tightly sealed by a light transmissive material 309 made of glass, a light transmissive resin, or the like. A water repellent material 303, also called a water repellent coating, is attached to a bottom surface formed by the light transmissive material 309 and the dielectric films 304 on the inner walls of the container 310.
Second electrodes 307 having a ring shape or the like are placed on the other edge (lid side edge) of the container 310 in contact with the conductive liquid material 301. The other edge is fluid-tightly sealed by a light transmissive material 308 made of glass, a light transmissive resin, or the like through a hydrophilic material 306, also called a hydrophilic coating. In this case, edges of the ring-shaped second electrodes 307 are formed to extend on an outer periphery of the light transmissive material 308.
Edges of the first electrodes 305 on an inner surface of the container 310 are formed to separate the first electrodes 305 from the second electrodes 307. In the example shown in the figure, the dielectric films 304 are similarly formed.
In such a configuration, appropriate voltage is applied between the first electrodes 305 and the second electrodes 307 by a voltage application unit 311 to change a curvature of an interface between the conductive liquid material 301 and the insulating liquid material 302. Accordingly, it is possible to change a lens effect on incident light indicated by an arrow Li from outside the light transmissive material 309 and change a focal length.
More specifically, when voltage is not applied to the first and second electrodes 305 and 307, the interface between the conductive liquid material 301 and the insulating liquid material 302 forms part of a spherical surface having a certain radius, as shown in FIG. 1A, by having a balance in surface tension of the packed liquid materials 301 and 302 and inner wall surfaces of the container 310. For example, when salt-containing water is used for the conductive liquid material 301 and silicone oil is used for the insulating liquid material 302, the salt water, that is, the conductive liquid material 301 forms convexity of the interface as shown in FIG. 1A. Here, a contact angle of the conductive liquid material 301 is indicated as θ(0).
When applying voltage between the first electrodes 305 and the second electrodes 307 by the voltage application unit 311, “wettability” of the conductive liquid material 301 to the inner wall surface of the container 310 is improved (this phenomenon is called electrowetting). Thus, the interface between the liquid materials 301 and 302 is changed to increase a radius of curvature of the interface and reduce a contact angle θ(V), for example, as shown in FIG. 1B.
When light is incident from the light transmissive material 309 as indicated by the arrow Li and emitted from the other light transmissive material 308, a lens effect is generated by a difference in refractive index between the liquid materials 301 and 302 and an interface curvature. Furthermore, the liquid interface curvature is changed due to electrowetting by applying voltage, so that a focal length of the lens is changed.
The varifocal lens utilizing such an electrowetting phenomenon is advantageous in that: current is essentially not flown in the varifocal lens except when discharging, causing power consumption extremely small; and the varifocal lens does not have a mechanical drive and thus has a life longer than that of a varifocal lens of the related art moved by a motor or the like. Further, the varifocal lens may save space and provide an auto-focus mechanism with a simple configuration, since the lens does not have to include a motor.